The photoconductive process in electrophotographic sensitive materials consists of (1) a step of generating electric charges by exposure and (2) a step of transporting the electric charges.
An example of carrying out steps (1) and (2) by means of a single substance includes a selenium electrophotographic sensitive plate, which is well known. On the other hand, an example of carrying out steps (1) and (2) by means of different substances, respectively, involves using a combination of amorphous selenium and poly-N-vinylcarbazole, which has been well known. The process of carrying out steps (1) and (2) by means of different substances, respectively, has advantages such as that the choices of selecting materials used for the electrophotographic sensitive materials is expanded, by which electrophotographic properties, such as sensitivity and acceptant electric potential, etc., of the sensitive materials, are improved and that materials suitable for production of a coating film of the sensitive material can be selected from a broad range of possibilities.
Hitherto, inorganic substances such as selenium, cadmium sulfide, zinc oxide, etc. have been used as the photoconductive material in the electrophotographic sensitive material used in electrophotographic processes.
As has been disclosed in U.S. Pat. No. 2,297,691, by Carlson, in the electrophotographic process, a photoconductive material comprising a base coated with a substance which is insulator in the dark, electric resistance of which varies corresponding to exposure by imagewise exposure to light, is used. This photo-conductive material is generally electrically charged in the dark after being subjected to dark adaptation for a suitable period of time. The material is then imagewise exposed to light in the form of a radiation pattern, which has the effect of reducing surface electric charges corresponding to the relative energy of the radiation pattern. The surface electric charges or electrostatic latent images remaining on the surface of the photoconductive layer (photosensitive layer) are then brought into contact with a suitable electroscopic indication material, namely, a toner, to form visible images. The toner is allowed to adhere to the surface of the sensitive layer corresponding to the electric charge pattern, whether the toner is contained in an insulating liquid or in a dry carrier. The indication material adhered on the surface can be fixed by known means such as by heat, pressure or a vapor of a solvent. Further, the electrostatic latent images can be transcribed onto a second base (for example, paper, film, etc.). Likewise, it is possible to develop the electrostatic latent images transcribed on the second base.
Basic characteristics required for the electrophotographic sensitive materials in such an electrophotographic process include the following: (1) the sensitive material can be electrically charged in the dark so as to have a suitable electric potential, (2) the degree of disappearance of electric charges is small in the dark, and (3) the electric charges can be rapidly dispersed by light exposure.
The above described inorganic substances used hitherto have various drawbacks at the same time they have a lot of advantages. For example, selenium, which is widely used at the present time, sufficiently satisfies the above described requirements (1) through (3). However, it has drawbacks in that the cost of production is high because of the severe conditions required for production, in that it is difficult to form into a belt because of lack of elasticity, and that it is necessary to pay attention to handling because it is sensitive to heat and mechanical shock. Cadmium sulfide and zinc oxide have been used as a sensitive material by dispersing in a resin binder but they can not be repeatedly used, because they have mechanical drawbacks with respect to smoothness, hardness, tensile strength, antifriction properties, etc.
In recent years, various electrophotographic sensitive materials using organic substances have been proposed, in order to overcome the drawbacks of the inorganic substances, and some of them have been put to practical use. Examples include a sensitive material comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one (U.S. Pat. No. 3,484,237), a sensitive material composed of poly-N-vinylcarbazole sensitized with a pyrilium dyestuff (Japanese Patent Publication No. 25658/73), a sensitive material comprising an organic pigment as a main component (Japanese Patent Application (OPI) No. 37543/72) and a sensitive material comprising an eutectic complex composed of a dye and a resin (Japanese Patent Application (OPI) No. 10735/72). Although these sensitive materials are believed to have a high practical value because of having excellent characteristics, the fact is that there is no organic substance which completely satisfies the above described requirements for the sensitive materials in the electrophotographic process.
As a result of carrying out studies on photoconductive substances, it has now been found that hydrazone compounds represented by the formula (I), as described hereinafter, effectively function as photoconductive substances for the electrophotographic sensitive materials, and excellent as charge transport materials.
Examples of using hydrazone compounds in electrophotographic sensitive materials have been described in U.S. Pat. No. 3,717,462 (corresponding to Japanese Patent Publication 8137/73), Japanese Patent Application (OPI) No. 59143/79 (corresponding to U.S. Pat. No. 4,150,987) and Japanese Patent Applications (OPI) No. 52063/80 and 52064/80, etc. All of these examples are condensed polynuclear compounds or N-alkylamino substituted compounds.
Electrophotographic sensitive materials having a photosensitive layer containing N-arylamino substituted compounds have already been found and proposed in Japanese Patent Application (OPI) No. 85495/80. In the sensitive materials, oxidation caused by ozone generating by corona discharging, which is a fatal fault in the sensitive materials of the prior art, stability to heat and light, and dark decay, etc. are remarkably improved and resulted in forming electrophotographic sensitive materials which have high sensitivity, low residual electric potential, residual electric potential causing generally fog, narrow variation of the residual electric potential and the sensitivity even when repeatedly using the materials and excellent durability. However, a continuing need exists for improved electrophotographic sensitive materials having excellent characteristics.